1. Field of the Invention
The present invention relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device that stores data in a non-volatile manner using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and is capable of random access to each thin film magnetic element.
In particular, recent announcement shows that significant progress in performance of the MRAM device is achieved by using thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as xe2x80x9cA 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cellxe2x80x9d, ISSCC Digest of Technical Papers, TA7.2, February, 2000, and xe2x80x9cNonvolatile RAM based on Magnetic Tunnel Junction Elementsxe2x80x9d, ISSCC Digest of Technical Papers, TA7.3, February, 2000.
FIG. 83 is a schematic diagram showing the structure of a memory cell having a magnetic tunnel junction (hereinafter, also simply referred to as xe2x80x9cMTJ memory cellxe2x80x9d).
Referring to FIG. 83, the MTJ memory cell includes a magnetic tunnel junction MTJ whose resistance value varies according to the storage data level, and an access transistor ATR. The access transistor ATR is formed from a field effect transistor, and is coupled between the magnetic tunnel junction MTJ and the ground voltage Vss.
For the MTJ memory cell are provided a write word line WWL for instructing a data write operation, a read word line RWL for instructing a data read operation, and a bit line BL serving as a data line for transmitting an electric signal corresponding to the storage data level in the data read and write operations.
FIG. 84 is a conceptual diagram illustrating the data read operation from the MTJ memory cell.
Referring to FIG. 84, the magnetic tunnel junction MTJ has a magnetic layer FL having a fixed magnetic field of a fixed direction (hereinafter, also simply referred to as xe2x80x9cfixed magnetic layer FLxe2x80x9d), and a magnetic layer VL having a free magnetic field (hereinafter, also simply referred to as xe2x80x9cfree magnetic layer VLxe2x80x9d). A tunnel barrier TB formed from an insulator film is provided between the fixed magnetic layer FL and the free magnetic layer VL. According to the storage data level, either a magnetic field of the same direction as that of the fixed magnetic layer FL or a magnetic field of the direction different from that of the fixed magnetic layer FL has been written to the free magnetic layer VL in a non-volatile manner.
In reading the data, the access transistor ATR is turned ON in response to activation of the read word line RWL. As a result, a sense current Is flows through a current path formed by the bit line BL, magnetic tunnel junction MTJ, access transistor ATR and ground voltage Vss. The sense current Is is supplied as a constant current from a not-shown control circuit.
The resistance value of the magnetic tunnel junction MTJ varies according to the relative relation of the magnetic field direction between the fixed magnetic layer FL and the free magnetic layer VL. More specifically, in the case where the fixed magnetic layer FL and the free magnetic layer VL have the same magnetic field direction, the magnetic tunnel junction MTJ has a smaller resistance value as compared to the case where both magnetic layers have different magnetic field directions.
Accordingly, in the data read operation, a voltage change produced at the magnetic tunnel junction MTJ due to the sense current Is varies according to the magnetic field direction stored in the free magnetic layer VL. Thus, by starting supply of the sense current Is with the bit line BL precharged to a high voltage, the storage data level in the MTJ memory cell can be read by monitoring a voltage level change on the bit line BL.
FIG. 85 is a conceptual diagram illustrating the data write operation to the MTJ memory cell.
Referring to FIG. 85, in the data write operation, the read word line RWL is inactivated, and the access transistor ATR is turned OFF. In this state, a data write current for writing a magnetic field to the free magnetic layer VL is applied to the write word line WWL and the bit line BL. The magnetic field direction of the free magnetic layer VL is determined by combination of the respective directions of the data write current flowing
FIG. 86 is a conceptual diagram illustrating the relation between the respective directions of the data write current and the magnetic field in the data write operation.
Referring to FIG. 86, a magnetic field Hx of the abscissa indicates the direction of a magnetic field H(WWL) produced by the data write current flowing through the write word line WWL. A magnetic field Hy of the ordinate indicates the direction of a magnetic field H(BL) produced by the data write current flowing through the bit line BL.
The magnetic field direction stored in the free magnetic layer VL is updated only when the sum of the magnetic fields H(WWL) and H(BL) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetic field direction stored in the free magnetic layer VL is not updated when a magnetic field corresponding to the region inside the asteroid characteristic line is applied.
Accordingly, in order to update the storage data of the magnetic tunnel junction MTJ by the data write operation, a current must be applied to both the write word line WWL and the bit line BL. Once the magnetic field direction, i.e., the storage data, is stored in the magnetic tunnel junction MTJ, it is held therein in a non-volatile manner until a new data write operation is conducted.
The sense current Is flows through the bit line BL in the data read operation. However, the sense current Is is generally set to a value that is smaller than the above-mentioned data write current by about one or two orders of magnitude. Therefore, it is less likely that the storage data in the MTJ memory cell is erroneously rewritten during the data read operation due to the sense current Is.
The above-mentioned technical documents disclose a technology of forming an MRAM device, a random access memory, having such MTJ memory cells integrated on a semiconductor substrate.
FIG. 87 is a conceptual diagram showing the MTJ memory cells arranged in rows and columns in an integrated manner.
Referring to FIG. 87, with the MTJ memory cells arranged in rows and columns on the semiconductor substrate, a highly integrated MRAM device can be realized. FIG. 87 shows the MTJ memory cells arranged in n rows by m columns (where n, m is a natural number).
As described before, the bit line BL, write word line WWL and read word line RWL must be provided for each MTJ memory cell. Accordingly, n write word lines WWL1 to WWLn, n read word lines RWL1 to RWLn, and m bit lines BL1 to BLm are required for the nxc3x97m MTJ memory cells.
Thus, the MTJ memory cells are generally provided with the independent word lines for the read and write operations.
FIG. 88 is a structural diagram of the MTJ memory cell provided on the semiconductor substrate.
Referring to FIG. 88, the access transistor ATR is formed in a p-type region PAR of the semiconductor main substrate SUB. The access transistor ATR has source/drain regions (n-type regions) 110, 120 and a gate 130. The source/drain region 110 is coupled to the ground voltage Vss through a metal wiring formed in a first metal wiring layer M1. A metal wiring formed in a second metal wiring layer M2 is used as the write word line WWL. The bit line BL is provided in a third metal wiring layer M3.
The magnetic tunnel junction MTJ is provided between the second metal wiring layer M2 of the write word line WWL and the third metal wiring layer M3 of the bit line BL. The source/drain region 120 of the access transistor ATR is electrically coupled to the magnetic tunnel junction MTJ through a metal film 150 formed in a contact hole, the first and second metal wiring layers M1 and M2, and a barrier metal 140. The barrier metal 140 is a buffer material for providing electrical coupling between the magnetic tunnel junction MTJ and the metal wirings.
As described before, the MTJ memory cell is provided with the read word line RWL independently of the write word line WWL. In addition, in the data write operation, a data write current for generating a magnetic field equal to or higher than a predetermined value must be applied to the write word line WWL and the bit line BL. Accordingly, the bit line BL and the write word line WWL are each formed from a metal wiring.
On the other hand, the read word line RWL is provided in order to control the gate voltage of the access transistor ATR, and a current need not be actively applied to the read word line RWL. Accordingly, from the standpoint of the improved integration degree, the read word line RWL is conventionally formed from a polysilicon layer, polycide structure, or the like in the same wiring layer as that of the gate 130 without providing an additional independent metal wiring layer.
As described in connection with FIG. 84, the data read operation of the MTJ memory cell is conducted based on the voltage change caused by the sense current (Is in FIG. 84) supplied to the magnetic tunnel junction MTJ serving as a resistive element. This voltage change cannot be quickly produced with a large RC (resistance-capacitance) time constant of the sense current path, making it impossible to increase the data read operation speed.
Moreover, as shown in FIG. 86, the data write operation is conducted based on the relation between the applied magnetic field and the asteroid characteristic line provided as a threshold. Accordingly, variation in asteroid characteristic line as produced in manufacturing the memory cells results in variation in data write margin to the memory cell.
FIG. 89 is a conceptual diagram illustrating the effects of the manufacturing variation on the data write margin.
Referring to FIG. 89, the design value of the asteroid characteristic line is denoted with ASd. It is now assumed that the asteroid characteristic line of the memory cell is deviated from the design value, as shown by ASa or ASb.
For example, in the MTJ memory cell having the asteroid characteristic line ASb, the data cannot be written even if the data write current according to the design value is supplied for application of the data write magnetic field.
On the other hand, in the MTJ memory cell having the asteroid characteristic line ASa, the data is written even if the data write magnetic field smaller than the design value is applied. As a result, the MTJ memory cell having such characteristics is extremely susceptible to the magnetic noise.
Such manufacturing variation in asteroid characteristic line may further be increased as the memory cells are miniaturized for improved integration. Accordingly, in order to ensure the manufacturing yield, there is a need not only for development of the manufacturing technology that reduces the manufacturing variation in asteroid characteristic line, but also for the adjustment technology for ensuring an appropriate data write margin corresponding to the variation in asteroid characteristic line.
Moreover, as described in connection with FIGS. 85 and 86, a relatively large data write current must be supplied to the bit line BL and the write word line WWL in the data write operation. As the data write current is increased, the current density in the bit line BL and the write word line WWL is also increased, which may possibly cause a phenomenon called electromigration.
Electromigration may cause disconnection or short-circuit of the wirings, thereby possibly degrading the operation reliability of the MRAM device. Moreover, an increased data write current may possibly produce a considerable amount of magnetic noise. It is therefore desirable to realize the structure capable of writing the data with a smaller data write current.
As described in connection with FIGS. 87 and 88, a large number of wirings are required to write and read the data to and from the MTJ memory cell, making it difficult to reduce the area of the memory array integrating the MTJ memory cells, and thus the chip area of the MRAM device.
An MTJ memory cell using a PN junction diode as an access element instead of the access transistor is known as a memory cell structure capable of achieving improved integration over the MTJ memory cell shown in FIG. 83.
FIG. 90 is a schematic diagram showing the structure of the MTJ memory cell using the diode.
Referring to FIG. 90, the MTJ memory cell using the diode includes a magnetic tunnel junction MTJ and an access diode DM. The access diode DM is coupled between the magnetic tunnel junction MTJ and the word line WL. Herein, the direction from the magnetic tunnel junction MTJ toward the word line WL is the forward direction. The bit line BL extending in such a direction that crosses the word line WL is coupled to the magnetic tunnel junction MTJ.
In the MTJ memory cell using the diode, the data write operation is conducted with the data write current being supplied to the word line WL and the bit line BL. As in the case of the memory cell using the access transistor, the direction of the data write current is set according to the write data level.
On the other hand, in the data read operation, the word line WL corresponding to the selected memory cell is set to the low voltage (e.g., ground voltage Vss) state. By precharging the bit line BL to the high voltage (e.g., power supply voltage Vcc) state, the access diode DM is rendered conductive, allowing the sense current Is to be supplied through the magnetic tunnel junction MTJ. The word lines WL corresponding to the non-selected memory cells are set to the high voltage state. Therefore, the corresponding access diodes DM are retained in the OFF state, and no sense current Is flows therethrough.
Thus, the data read and write operations can be conducted also in the MTJ memory cell using the access diode.
FIG. 91 is a structural diagram of the MTJ memory cell of FIG. 90 provided on the semiconductor substrate.
Referring to FIG. 91, the access diode DM is formed on the semiconductor substrate SUB from an N-type region NWL formed from, e.g., an N-type well, and a P-type region PRA formed thereon.
The N-type well NWL, which corresponds to the cathode of the access diode DM, is coupled to the word line WL provided in the metal wiring layer M1. The P-type region PRA, which corresponds to the anode of the access diode DM, is electrically coupled to the magnetic tunnel junction MTJ through the barrier metal 140 and the metal film 150. The bit line BL is provided in the metal wiring layer M2 so as to be coupled to the magnetic tunnel junction MTJ. Thus, by replacing the access transistor with the access diode, the MTJ memory cell that is advantageous in terms of improvement in integration degree can be obtained.
The data write current flows through the word line WL and the bit line BL in the data write operation. This causes a voltage drop on these lines. Such a voltage drop may turn ON the PN junction of the access diode DM of at least one of the MTJ memory cells that are not selected for the data write operation. As a result, a current may unexpectedly flow through the MTJ memory cell, causing an erroneous data write operation.
Thus, the conventional MTJ memory cell using the access diode is advantageous in terms of improved integration, but is problematic in view of the stability of the data write operation.
It is an object of the present invention to increase the data write speed in an MRAM device including MTJ memory cells.
It is another object of the present invention to provide the structure capable of easily adjusting the amount of data write current so as to assure a predetermined data write margin in the MRAM device including the MTJ memory cells, by compensating for variation in magnetic characteristics due to manufacturing variation.
It is a further object of the present invention to achieve improvement in operation reliability as well as suppression of magnetic noise in the MRAM device including the MTJ memory cells, by reducing the data write current.
It is a still further object of the present invention to provide the MTJ memory cell structure capable of improved integration and providing excellent operation reliability.
It is a yet further object of the present invention to suppress the chip area of the MRAM device including the MTJ memory cells arranged in an array, by improving the freedom of layout as well as reducing the number of wirings required for the entire memory array.
In summary, according to the present invention, a thin film magnetic memory device includes a memory array, a plurality of first bit lines, a plurality of read word lines, a first read data line, a read gate circuit, and a data read circuit. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells has either a first or second resistance value according a storage data level thereof. The plurality of first bit lines are provided corresponding to the respective columns of the magnetic memory cells. The plurality of read word lines are provided corresponding to the respective rows of the magnetic memory cells, for electrically coupling the magnetic memory cells corresponding to an addressed row between the plurality of first bit lines set to a first voltage and a second voltage (Vss), respectively, so as to pass a data read current through the magnetic memory cells. The first read data line transmits read data. The read gate circuit sets a voltage of the first read data line according to a voltage on one of the plurality of first bit lines that corresponds to an addressed column. The data read circuit sets a level of the read data according to the voltage on the first read data line.
Therefore, a primary advantage of the present invention is that the data read speed can be increased by rapidly producing a voltage change on the first bit line by conducting the data read operation with a reduced RC constant of the data read current path, without supplying any data read current to the first read data line.
According to another aspect of the present invention, a thin film magnetic memory device having a normal operation mode and a test mode includes a memory array, a plurality of write word lines, a write word line driver, a data write circuit, and a plurality of bit line pairs. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells has a different resistance value according to a level of storage data written when a data write magnetic field applied by first and second data write currents is larger than a predetermined magnetic field. The plurality of write word lines are provided corresponding to the respective rows of the magnetic memory cells, and selectively activated according a row selection result in a data write operation. The write word line driver supplies the first data write current to the activated word line in an amount corresponding to a voltage level on a first control node. The data write circuit supplies the second data write current in the data write operation in an amount corresponding to a voltage level on a second control node. The plurality of bit lines are provided corresponding to the respective columns of the magnetic memory cells, and selectively connected to the data write circuit according to a column selection result in the data write operation. At least one of the write word line driver and the data write circuit includes an input terminal for externally setting the voltage level of a corresponding one of the first and second control nodes in the test mode.
Accordingly, in the test mode, at least one of the first and second data write currents can be set from the outside. Thus, the manufacturing variation in magnetic characteristics of the MTJ memory cells can be compensated for, whereby the adjustment testing of the data write current amount for appropriately ensuring a data write margin can be facilitated.
According to a further aspect of the present invention, a thin film magnetic memory device includes a memory array, a plurality of bit lines, a plurality of write word lines, and a coupling circuit. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells includes a magnetic storage portion having a different resistance value according to a level of storage data written when a data write magnetic field applied by first and second data write currents is larger than a predetermined magnetic field. The plurality of bit lines are provided corresponding to the respective columns of the magnetic memory cells, for passing the first data write current therethrough. The plurality of write word lines are provided corresponding to the respective rows of the magnetic memory cells, and selectively activated according an address selection result so as to pass the second data write current therethrough in a data write operation. Each of the write word lines includes first and second sub write word lines respectively formed in first and second metal wiring layers with the magnetic storage portions interposed therebetween in a vertical direction on a semiconductor substrate. The coupling circuit electrically couples the first and second sub write word lines to each other. The second data write current flows as a reciprocating current through the first and second sub write word lines electrically coupled to each other by the coupling circuit.
Thus, since the data write current flows as a reciprocating current through the first and second bit lines that are electrically coupled to each other, data write magnetic fields acting in the same direction can be generated in the magnetic storage portion. This reduces the amount of data write current required to generate a data write magnetic field of the same strength. As a result, reduced power consumption of the MRAM device, improved operation reliability resulting from the reduced current density of the bit line, and also reduced magnetic field noise in the data write operation can be realized.
According to a still further aspect of the present invention, a thin film magnetic memory device includes a memory array, a plurality of read word lines, a plurality of write word lines, and a plurality of bit lines. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells includes a magnetic storage portion having a different resistance value according to a level of storage data written when a data write magnetic field applied by first and second data write currents is larger than a predetermined magnetic field. The plurality of read word lines are provided corresponding to the respective rows of the magnetic memory cells, and are driven to a first voltage according to a row selection result in a data read operation. The plurality of write word lines are provided corresponding to the respective rows, and are selectively activated according an address selection result so as to pass the first data write current therethrough in a data write operation. The plurality of bit lines are provided corresponding to the respective columns of the magnetic memory cells so as to extend in such a direction that crosses the plurality of write word lines, and are each coupled to the magnetic storage portions. One of the plurality of bit lines that is selected according to an address selection result passes therethrough a data read current and the second data write current in the data read operation and the data write operation, respectively. Each of the magnetic memory cells further includes a rectifying element connected between the corresponding magnetic storage portion and the corresponding read word line.
Such a magnetic memory cell using the rectifying element is advantageous in terms of improved integration, and the OFF state of the rectifying element can be reliably maintained in the non-selected magnetic memory cells. As a result, the improved integration can be achieved as well as the operation reliability can be ensured.
According to a yet further aspect of the present invention, a thin film magnetic memory device includes a memory array, a plurality of read word lines, a plurality of write word lines, a plurality of write data lines, and a plurality of read data lines. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells includes a magnetic storage portion having a different resistance value according to a level of storage data written when a data write magnetic field applied by first and second data write currents is larger than a predetermined magnetic field, and a memory cell selection gate for passing a data read current therethrough into the magnetic storage portion in a data read operation. The plurality of read word lines are provided corresponding to the respective rows of the magnetic memory cells, for actuating the corresponding memory cell selection gate according to an address selection result in the data read operation. The plurality of write word lines are provided corresponding to the respective columns of the magnetic memory cells, and are selectively driven to an active state according to an address selection result so as to pass the first data write current therethrough in a data write operation. The plurality of write data lines are provided corresponding to the respective rows, for passing the second data write current therethrough in the data write operation. The plurality of read data lines are provided corresponding to the respective columns, for passing the data read current therethrough in the data read operation. Adjacent magnetic memory cells share a corresponding one of at least one of the plurality of write word lines, the plurality of read word lines and the plurality of data lines.
Thus, the read word lines and the write word lines are provided corresponding to the rows and columns of the magnetic memory cells, respectively, and respective circuits for selectively driving the read word lines and the write word lines are provided independently. Accordingly, the freedom of layout can be improved. Moreover, at least one of the write word lines, read word lines, write data lines, and read data lines are shared between corresponding adjacent memory cells, whereby a wiring pitch in the memory array can be widened. As a result, the integration degree of the MRAM device can be improved.
According to a yet further aspect of the present invention, a thin film magnetic memory device includes a memory array, a plurality of read word lines, a plurality of write data lines, a plurality of common lines, and a current control circuit. The memory array includes a plurality of magnetic memory cells arranged in rows and columns. Each of the plurality of magnetic memory cells includes a magnetic storage portion having a different resistance value according to a level of storage data written when a data write magnetic field applied by first and second data write currents is larger than a predetermined magnetic field, and a memory cell selection gate for passing a data read current (Is) therethrough into the magnetic storage portion in a data read operation. The plurality of read word lines are provided corresponding to the respective rows of the magnetic memory cells, for actuating the corresponding memory cell selection gate according to an address selection result in the data read operation. The plurality of write data lines are provided corresponding to the respective rows, for passing the first data write current therethrough in a data write operation. The plurality of common lines are provided corresponding to the respective columns of the magnetic memory cells. Each of the plurality of common lines selectively receives supply of the data read current according to the address selection result in the data read operation. Each of the plurality of common lines is selectively driven to a first voltage (Vcc) for passing the second data write current therethrough according to the address selection result in the data write operation. The current control circuit electrically couples and disconnects each of the common lines to and from a second voltage (Vss) in the data write operation and the data read operation, respectively. The second voltage is different from the first voltage. Adjacent magnetic memory cells share a corresponding one of at least one of the plurality of write data lines, the plurality of read word lines and the plurality of common lines.
Thus, each common line functions as a read data line in the data read operation, and as a write word line in the data write operation, whereby the number of wirings can be reduced. A circuit for selectively driving the read word lines and a circuit for selectively driving the common lines in the data write operation, i.e., the common lines functioning as write word lines, are provided independently, whereby the freedom of layout can be improved. Moreover, at least one of the read word lines, write data lines and common lines are shared between corresponding adjacent memory cells, whereby a wiring pitch in the memory array can be widened. As a result, the integration degree of the MRAM device can be improved.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.